Subsea production for many oil companies is projected to increase significantly in the next 5-10 years. In addition, offshore fields are being exploited in deeper and deeper waters. However, producing from floating production, storage, and offloading vessel (FPSO) presents many challenges, which increase as the water depth increases.
Referring to FIG. 1, produced fluids are often carried from a wellhead 10 on the seabed 20 to an FPSO 30 through flow lines such as flexible risers 100. Additionally, a riser clamp 40 and tethers 50 may be employed to retain the end of the flexible riser 100 to the seabed 20, while buoyancy modules 60 may be connected to various positions along the flexible riser 100 and a bend stiffener 80 may be included at the connection to FPSO 30 so as to allow for surface movement of the FPSO 30 and movements due to wave action without putting undue stress upon the flexible riser 100. As labeled in FIG. 1, the bends created in the flexible riser 100 by the buoyancy modules 60 are commonly referred to as “hog bends” at high points and “sag bends” at low points.
Flexible risers 100 bring many advantages allowing produced fluids to flow from the fixed seabed wellhead 10 to the FPSO 30, which will move with tidal and wave action. Additionally, flexible risers 100 may be manufactured in long continuous lengths that allow for a simpler and more efficient installation. The use of flexible risers 100 is well documented and known to one of ordinary skill in the art.
Referring to FIG. 2, a typical flexible structure 100 consists of many layers, each of which plays a different role from providing structural strength to providing isolation between the inside bore 102, which carries producing fluids, from the outside sea water. The steel reinforcing layers (armours 108 and pressure vault 106) are contained within a very confined environment called the annulus, which is located between the innermost sheath that surrounds the inside bore 102, such as inner polymer sheath 104 and the outermost sheath that contacts the sea water, such as external polymer sheath 110. The inner polymer sheath 104 is the barrier to the conveyed production fluids and the external polymer sheath protects 110 against the seawater environment.
If water is present in the annulus, then the longer term integrity of the flexible riser 100 may be compromised due to corrosion. It should be noted that although the inner sheath 104 and outer sheath 110 are impermeable, under high temperature and pressure conditions small amounts of gases can permeate through the inner sheath 104. Corrosive gases are often present in production fluids (e.g. H2S, CO2, and water vapor), plus hydrocarbons such as CH4, and can diffuse through the inner sheath and accumulate in the annular space. This results in a corrosive environment in contact with the carbon steel members, which can significantly reduce the life of the flexible riser 100.
In addition, it is possible that the outer sheath 110 may be damaged during installation, which can allow a slow ingress of seawater over time. If the outer sheath 110 becomes seriously breached, the annulus may become flooded. Also, a slow diffusion of water through the outer sheath 110 is also possible. In all such cases, water enters the annulus causing corrosion of the steel wire structures, which can result in premature failure of the flexible riser 100.
The failure of a flexible riser 100 can be very costly due to lost production and associated installation services, which may vary widely depending on the availability of such services. Also, the failure may result in catastrophic damage to the environment. However, if failures are detected early and monitored, repair or replacement can be scheduled in order to significantly reduce the risk of environmental damage and minimize the down-time of production.
Determination of the presence of liquid in the annulus is presently achieved by periodically monitoring the vented gas flow rate from the annulus and vacuum testing. If water collection occurs, then the gas displaced thereby is vented at surface. However, this approach is not very accurate and small amounts of water intrusion are difficult to detect. In addition, flexible risers 100 are often installed with a buoyancy modules 22 in the middle and, if water collects in the low lying section around a sag bend, gas may become trapped and not vent at the surface. As a result, pressure within the annulus may increase to the point where the outer sheath 110 is ruptured, thereby causing catastrophic failure.
Other approaches to measure the pressure in an annulus have been developed. See, for example, EP 1492936B1 (hereinafter “Technip”). Technip describes a method and device that measures the pressure in the annulus at the bottom of the flexible riser along with the hydrostatic pressure of the seawater column and compares the difference between the two measurements. This approach requires that specific measurements both inside the annulus and outside the flexible riser are made. Also, the method requires a special measurement component at the end of the flexible riser and requires electrical wiring to run to surface with connections between this wiring and the measurement sub. Also, this method and device would not likely be able to identify water collection in low lying areas around a sag bend.
Another existing method involves periodically pulling a vacuum on the vent lines at the surface that connect to the annulus. The degree to which a vacuum can be held is used to give indication of any leaks in the inner or outer sheaths. In practice, this method is generally recognized as difficult to control and not very reliable.